The present invention relates to a method for producing a pattern of hydrophilic and hydrophobic regions on a surface which, in a first, essentially unpatterned state, features a polymer material containing imide groups. Moreover, the present invention relates to a printing form, in particular, for use as printing form in offset printing, featuring a surface for printing.
To express it in simplified terms, lithographic printing is based on making use of the immiscibility of oil and water on a surface, the so-called “printing form”, the image-forming regions retaining the lipophilic (hydrophobic) solution or the ink or color and the non-image forming regions of the printing surface retaining the water or the hydrophilic solution. When the printing surface, which has been prepared in a suitable manner, is wetted with hydrophilic and lipophilic substances or solutions, in particular water and ink or color, then the non-image regions preferably retain the hydrophilic substance or solution and repel the lipophilic substances while the image regions take up the lipophilic solution or ink or color and repel the hydrophilic substances. Consequently, the lipophilic substance is then transferred in a suitable manner onto the surface of a material on which to fix the image, for example, paper, cloth, polymers, or the like.
For many years, aluminum has been used as material for printing forms. Usually, the aluminum is initially subjected to a graining process and then to a subsequent anodizing process. The anodization serves to provide an oxide layer whose adherence is improved by the granularity. The granularity boosts the hydrophilic properties of the background of the printing plate. In the anodizing process, usually a strong acid, such as sulfuric or phosphoric acid, is used to subsequently make the surface hydrophilic by a further process such as in a thermal siliconization process or the so-called “electrosiliconization”.
To produce an above described printing form, a large number of radiation-sensitive materials are known which, in the use of the lithographic printing method, are suitable for generating images in that they provide an image region which is usable for printing upon exposure and possibly required development and fixation. For this purpose, it is possible to use, for example, photopolymerizable substances.
The above described arrangement is subjected to an imaging exposure by supplying energy in a locally selective manner. This can be accomplished by exposure with UV light through a mask, or else by direct writing with a laser.
The lithographic printing forms of the type described above are usually treated with a developer solution which typically is an aqueous alkaline or basic solution containing organic additives.
For some time, efforts have been made to produce printing forms in connection with which a wet chemical developing method for producing the image can be dispensed with. To this end, it is possible to use oxide ceramics which exist, for example, in the form of coatings on a printing plate.
In European Patent Application EP 0 911 154 A1, the materials proposed for the plate surface are titanium dioxide (TiO2) and zirconium dioxide (ZnO2) which can be both pure or mixed with other metallic additives in different ratios. In the non-excited state, this surface is hydrophobic and capable of being transformed into a hydrophilic state by irradiation with ultraviolet light. The imaging is now accomplished in that the entire surface of the plate is illuminated with ultraviolet light and regions which are intended to carry ink during printing are covered with a mask or a film.
At least when using titanium dioxide layers as substrate, it turns out to be particularly disadvantageous that the titanium dioxide layers can indeed be switched with UV light but have a low stability with respect to the time characteristic of the switching. In the case of titanium dioxide layers, moreover, it turns out again and again that a sufficient switching or a sufficient shift, i.e., a sufficient flip from hydrophilic to hydrophobic is achievable only with inadequate intensity. Furthermore, the complete cleaning of the substrate upon the completion of the printing represents a problem in practice which is not to be underestimated.
U.S. Pat. No. 4,568,632 describes the patterning of polymer surfaces or polymer films containing at least one imide group in the corresponding monomer, whether in the main chain or a side chain of the polymer composed thereof. It discloses a method for etching or removing polyimide without chemical treatment steps. The polyimide is exposed to ultraviolet light having a wavelength shorter than 220 nm, for example, from an argon fluoride excimer laser so that a photocatalytic decomposition takes place during which volatile products are removed by suitable means. To support, in particular, to accelerate the process, the reaction is carried out in an atmosphere which contains oxygen. A patterning can be achieved, for example, by using a mask which is illuminated over a large area or by scanning the surface with an exposure beam for a spatially selective reaction. This patterning can be accomplished without significantly influencing the polyimide which remains at the surface. Therefore, a patterning of the surface into hydrophobic and hydrophilic regions which, in particular, would allow the patterned surface to be used for the printing process according to a lithographic method or an offset method cannot be achieved.